The present invention relates generally to automatic gain control in a radio frequency receiver. More particularly the invention relates to a method of controlling gain of an amplifier for received radio signals in a radio receiver, according to the preamble of claim 1. The invention also relates to a computer program, for instance in the form of a digital processor algorithm, according to claim 12, a computer readable medium according to claim 13 and an arrangement according to the preamble of claim 14.
A problem that arises in most radio communications receivers concerns the wide variation in power level of the radio signals received at the antenna. This variation is due to a variety of causes. For example, the distance between the transmitter and the receiver can vary considerably. Different transmitters may also utilise different power levels. Since, disregarding the influence of any screening objects, the received signal power decreases as the square of the distance to the transmitter, wide variations in received power level are likely to arise in many situations. Furthermore, these variations may occur very rapidly due to changes in the radio conditions. Movements of the receiver station and/or transmitter station or repositioning of objects between the stations are typical situations in which the conditions for the radio channel can change dramatically.
In radio design it is therefore common practice to include an automatic gain control (AGC) circuit in the receiver. The AGC circuit utilises feedback to maintain a fixed (or at least as stable as possible) signal power level within the receiver even though the signal level at the antenna varies widely. The AGC is achieved by using an amplifier whose gain can be controlled by an external signal, e.g. a voltage or a current.
In analogue receivers it is known to incorporate AGC circuits that operate on a down converted intermediate frequency (IF) signal, i.e. a signal component, which has been frequency transformed down from a received radio frequency (RF) signal and which is to be further frequency transformed down in a following frequency down conversion step.
Receivers in which the radio signal is digitally processed, in most cases after frequency down conversion, usually perform the AGC operation by digitally assisted processing. Thus, the AGC loop implies both analogue to digital conversion and digital to analogue conversion. For many of today's applications this gives a satisfying compensation for the power level variations in the received radio signals.
However, besides capable A/D- and D/A-converters, digital AGC also requires an amount of processing power, which in turn is correlated with power consumption and costs. For large signal bandwidths this effect becomes especially pronounced. Moreover, a large bandwidth places relatively demanding requirements on the A/D- and D/A-converters, particularly if a high digital resolution is necessary.
In a radio communication system for so-called bursty communication, for instance in the form of data packets, short pieces of information are passed between transmitter stations and receiver stations at irregular and generally unpredictable time instances. A particular station can, in most such systems, act both as a transmitter station and as a receiver station. With some exceptions, this means that every station in the system is a potential receiver of a radio message at any time. The station must therefore be capable of tuning its AGC circuit to received radio signals very rapidly.
The IEEE: 802.11a, 802.11b and ETSI: Hiperlan/2 constitute specific examples of wireless LAN protocols where extremely quick and accurate AGC-tuning is demanded. (IEEE=The Institute of Electrical and Electronics Engineers, ETSI=The European Telecommunications Standards Institute, LAN=Local Area Network, Hiperlan=High performance radio local area network). 802.11b specifies packet data exchange at speeds up to 11 Mbps/channel (under direct sequence modulation at 2.4 GHz) and Hiperlan/2 makes possible wireless access to the Internet and real time video services at speeds of 54 Mbps (at 5 GHz). In order to meet the hardest requirements of these standards a radio receiver must be capable of calibrating its receiver circuitry to the power level of received radio signals within 10 μs from start of transmission. This means that the AGC function must control the receiver amplifier gain to a suitable level within 10 μs or less.
However, it is both technically complicated and expensive to accomplish an AGC function involving digital signal level detection with sufficient accuracy and within such short time limits by utilising today's A/D converters, D/A converters and digital signal processors.